1. Field of the Invention
Embodiments discussed herein are directed to an apparatus and method for adjusting optical signals by adjusting signal light components with multiple wavelengths for each of the wavelengths.
2. Description of the Related Art
In WDM (Wavelength Division Multiplexer) transmission systems, when the number of optical wavelengths (accommodating ports) or span loss in transmission channels varies in an OADM (Optical Add-Drop Multiplexer), an optical amplifier or others, a VOA (Variable Optical Attenuator) is used in an optical signal level adjustment unit to keep a constant intensity (power) of an optical signal in order to maintain a certain communication quality.
FIG. 1 is a block diagram illustrating an exemplary conventional WDM transmission apparatus. In this illustration, an optical signal transmitted in an optical channel and multiplexed with wavelengths λ1 through λN is amplified in an optical amplifier 11 and then is wavelength-demultiplexed in a wavelength demultiplexer (DMUX) 12. After optical signals with different wavelengths are switched in an optical switch (OSW) 13, the optical signals are adjusted in respective optical signal level adjustment units 14-1 through 14-N to make the respective intensity (power) levels of the optical signals uniform. Then, the adjusted optical signals are wavelength-multiplexed in a wavelength multiplexer (MUX) 15, amplified in a transmission amplifier 16 and then supplied to an optical channel.
FIG. 2 is a block diagram illustrating an exemplary conventional optical signal level adjustment unit. In this illustration, an optical signal level adjustment unit includes a monitor (PD) unit 14a for detecting the intensity of an optical signal, a control (CNT) unit 14b for generating a control signal corresponding to the detected intensity of an optical signal and an adjustment (VOA) unit 14c. 
Japanese Laid-open Patent Publication No. 2004-140631 proposes that VOAs associated with respective multiple different wavelengths are controlled depending on output levels of the VOAs and analyzed levels of the respective wavelengths by monitoring wavelength-multiplexed light components of outputs of the VOAs associated with multiple different wavelengths.
Recent explosive growth of optical communications traffic has promoted larger-scale WDM transmission systems and demanded more compact circuits for adjusting power of optical signals.
In such a larger-scale WDM transmission system, however, the control unit 14b for level adjustment of optical signals must include a number of correction units 14b1 and VOA control unit circuits 14b2 corresponding to the number of wavelengths, resulting in a larger-scale circuit.